1. Field of the Invention
The present invention relates to the field of Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), and, more particularly, to range measurement corrections in such systems.
2. Description of the Related Art
Global Navigation Satellite Systems (GNSS) use a constellation of dispersed satellites with atomic clocks orbiting the Earth that transmit predictable signals at exact times. The modulation used by these signals and the data messages included enable the receivers to determine highly accurate navigational locations anywhere on the Earth. The receiver calculates its position by carefully measuring the time of arrival of the signals sent by several of the satellites. Each satellite continually transmits messages containing the time the message was sent, precise orbital information, and the general system health and approximate orbits of all the satellites. By calculating the difference between the broadcast “transmit time” and the received “time of arrival,” a time of propagation can be determined and transformed into a range using the speed of propagation “c”.
GNSS is considered a dual-use technology, namely, a technology that has significant civilian and military applications. Accordingly, for an example GNSS like the Global Positioning System (GPS), the satellites broadcast on precisely defined carrier frequencies with well-defined modulation. The GPS data and timing signals intended for everyone's use have a publicly-defined format contained in Interface Specification IS-GPS-200, and are unencrypted, while those timing signals intended for military use are not publicly defined and are encrypted and the military specific information content is also encrypted. The satellite employs a pseudorandom code, which is used to modulate the carrier frequency in order to transmit the precise time marks. The carrier frequencies are over 1 GHz, while the code rates are considerably lower. GPS chip rates are roughly 10 MHz for the military code and 1 MHz for the civilian code. Additionally, data messages containing satellite orbit, system health, and other necessary information are transmitted at even a lower rate of 50 bits per second.
Most conventional civilian navigational systems receive a GNSS signal through a single element fixed reception pattern antenna (FRPA) coupled to the receiver. Many military systems, however, use a multiple element controlled reception pattern antenna (CRPA) system to receive a GNSS signal. CRPA systems are much more resistant to the effects of intentional jamming of the GNSS frequencies than are FRPA systems and the signals from each of the elements can be coherently added to increase the carrier-to-noise-density ratio (C/NO) over that of a conventional FRPA type antenna for each received signal.
With GNSS, the receiver measures the transit time, using the precise time marks provided by the pseudorandom code, from a satellite and computes the distance to that satellite by multiplying the transit time by the speed of light. These distance computations are called “pseudoranges” since there is almost always a time difference between the atomic satellite clocks used to create the precise time marks and the receiver clocks used to decode the precise time marks. This clock error is common to all measurements since the atomic satellite clocks are all synchronized, and results in a common range error. This common range error is what forms what is often called a “pseudorange” from an absolute range. Other effects that give rise to range errors include atmospheric, multipath, and receiver and antenna hardware.
Geometric multilateration is used to pinpoint the receiver's location by combining these pseudoranges with the corresponding locations of the satellites, using the data from at least four different satellites. Four pseudoranges also allow determination of the clock bias associated with the common range error described above, which adds a fourth dimension of uncertainty, when trying to solve for the other three dimensions of a physical location. Nonetheless, other effects that contribute to range measurement errors still remain. Identifying and attempting to account for the multiple sources of errors is an important step to improving the accuracy of locations determined through GNSS.
Atmospheric (i.e., tropospheric and ionospheric) delays are usually the next most significant source of error. The Earth's atmosphere slows down the speed of the satellite transmissions. These errors can be compensated for in a number of ways. For instance, using satellites that are more directly overhead helps because their transmissions travel through less atmosphere than when using satellites closer to the horizon. In addition, having the satellites transmit on multiple frequencies helps mitigate the ionospheric induced errors since it is frequency-dependent, so can be mitigated by combining the measurements from the two frequencies into a single ionospheric free measurement. Finally, relative positioning systems, such as Differential GPS, use strategically placed monitor stations at exact locations to determine at any given time what the overall transmission delay (including effects like atmospheric) is for each satellite. These monitor stations then broadcast these delays to all nearby receivers, which then can make the corrections to each of the corresponding satellites.
There are still other effects, most notably receiver antenna hardware, which cannot be compensated through any of the above techniques. To the extent that such effects, such as directionally dependent group delay errors, are not common between different satellites (common errors disappear as part of the clock bias correction calculated when determining location), they can affect the accuracy of the resulting positional calculation. Multiple element receiver antennas add complexity to the mitigation of these non-common errors, because the directionally dependent group delay errors may differ between antenna elements. Each hardware path contributes a different delay to the overall measured time of reception. Accounting for these more complex differences helps systems using multiple element antennas achieve the same accuracy that single element antennas are capable of achieving.
Because the satellite signals are relatively weak, it is fairly straightforward to intentionally jam such signals, either by increasing the noise floor by transmitting broadband noise or by attempting to exceed the dynamic range of the receiver hardware with powerful narrowband signals. Additionally, since the satellite signal structure is so precisely defined and predictable, it can be spoofed by a transmission using the same frequencies and signal structure. This is unacceptable for military applications, so they rely on encrypted signals to thwart any spoofed transmissions, but are still susceptible to intentional interference on the same frequencies. Consequently, for military applications, there is a need to reduce the effect of jamming, so the CRPA system is sometimes used in place of the FRPA system.
Intentional interference is usually significantly stronger than actual satellite transmissions. CRPA systems can use techniques such as nulling (combining the signals received by the CRPA's elements in such a way as to make the jamming signal cancel itself out) or beam steering (combining the signals received by the CRPA's elements in such a way as to amplify the satellite signal) to overcome intentional jamming. Note that beam steering doesn't physically direct the antenna hardware, rather it uses phased array techniques to compensate for the phase of arrival difference caused by the different path length to each element from any satellite to make the signals from each antenna element phase coherent so they add together in amplitude. Also note that it is possible to perform nulling and beam steering at the same time.
For high quality GNSS pseudorange measurements, non-common receiver hardware induced errors and directionally dependent antenna induced errors must be compensated for to obtain desired accuracies for high precision GNSS requirements in certain applications. The problem of non-common receiver hardware induced errors is a significant issue in sensors that receive the Russian Global Navigation Satellite System (GLONASS), and may be mitigated by using an internally generated calibration signal to measure and eliminate receiver induced errors from the pseudorange measurements, as disclosed by Lennen (U.S. Pat. Nos. 5,949,372 and 6,266,007). The problem of directionally dependent antenna induced errors is straightforward to solve in FRPA GNSS sensors by subtracting the directionally dependent antenna errors from the pseudorange measurements, in a similar manner to the standard method of correcting GNSS carrier phase measurements for FRPA antenna induced errors that have existed for some years. See, for example, Gerald L. Mader, GPS Antenna Calibration at the National Geodetic Survey, the entire content of which is herein incorporated by reference. However, for the complex case of a GNSS receiver employing a CRPA and dynamic beam steering, the multiplicity of combinations of antenna element outputs makes compensation of directionally dependent antenna induced errors more difficult, as the simple subtraction that might be used for FRPA compensation does not work with a CRPA. Compensation of pseudorange measurements for such errors is a problem not addressed in previous GNSS CRPA beam steering sensors.
Therefore, with the conversion from the FRPA based systems to CRPA systems for GNSS applications, there is a need to better compensate for the effects of antenna element errors on pseudorange measurement errors.